Asbestos have been used as a heat-resistant sealing material, for example, since they are light in weight and have excellent heat resistance. However, use of asbestos is prohibited since it causes disorders of lungs. Therefore, instead of asbestos, ceramic fibers or the like have been used. It is thought that ceramic fibers or the like have excellent heat resistance which is equivalent to that of asbestos, and no health problem may occur as long as they are handled appropriately. However, there is a trend that a higher degree of safety is required. Under such circumstances, various bio-soluble fibers have been developed in order to realize bio-soluble fibers which do not cause or hardly causes health problems even if they are inhaled in a human body (see Patent Document 1, for example).
Like asbestos, conventional inorganic fibers are secondary processed into a shaped product or an unshaped product together with various binders or additives, and are used as a joint in a heat treating apparatus, a furnace such as an industrial furnace, an incinerator or the like, a joint which fills the gap of refractory tiles, insulating bricks, shell, refractory mortar or the like, a sealing material, a packing material, an insulating material, or the like. In many cases, the inorganic fibers in use are exposed to high temperatures, and they are required to have heat resistance.
Further, in many cases, alumina is used in a member of a furnace. There was a problem that fibers contained in a secondary-processed product react with the alumina, thereby causing the secondary product or the member to adhere and melt.